Mycobacterium is a genus of aerobic intracellular bacterial organisms that, upon infection of a host, survive within endosomal compartments of monocytes and macrophages. Human mycobacterial diseases include tuberculosis (caused by M. tuberculosis), leprosy (caused by M. leprae), Bairnsdale ulcers (caused by M. ulcerans), and various infections caused by M. marinum, M. kansasii, M. scrofulaceum, M. szulgai, M. xenopi, M. fortuitum, M. chelonae, M. haemophilum and M. intracellulare (see Wolinsky, Chapter 37 in Microbiology: Including Immunology and Molecular Genetics, 3rd Ed., Harper & Row, Philadelphia, 1980).
One third of the world's population harbors M. tuberculosis and is at risk for developing tuberculosis (TB). In immunocompromised patients, tuberculosis is increasing at a nearly logarithmic rate, and multidrug resistant strains are appearing. In addition, mycobacterial strains which were previously considered to be nonpathogenic strains (e.g., M. avium) have now become major killers of immunosuppressed AIDS patients. Moreover, current mycobacterial vaccines are either inadequate (such as the BCG vaccine for M. tuberculosis) or unavailable (such as for M. leprae) (Kaufmann, S., Microbiol. Sci. 4:324-328, 1987; U.S. Congress, Office of Technology Assessment, The Continuing Challenge of Tuberculosis, pp. 62-67, OTA-H-574, U.S. Government Printing Office, Washington, D.C., 1993).
Inhibiting the spread of tuberculosis requires effective vaccination and accurate, early diagnosis of the disease. Currently, vaccination with live bacteria is the most efficient method for inducing protective immunity. The most common mycobacterium employed for this purpose is Bacillus Calmette-Guerin (BCG), an avirulent strain of Mycobacterium bovis. However, the safety and efficacy of BCG is a source of controversy and some countries, such as the United States, do not vaccinate the general public.
Mycobacterium tuberculosis (Mtb)-specific CD4+ and CD8+ T cells are critical for the effective control of Mtb infection. In the mouse model, passive transfer of CD4+ T cells to sublethally irradiated animals renders them less susceptible to Mtb infection (Orme, J. Immunol. 140:3589-3593, 1988). Mice in which the gene(s) for CD4 (CD4−/−) or for MHC Class II molecules are disrupted, as well as wild-type mice depleted of CD4+ T cells, demonstrate increased susceptibility to Mtb infection (Flory et al., J. Leukoc. Biol. 51:225-229, 1992). In humans, human immunodeficiency virus-infected individuals are exquisitely susceptible to developing TB after exposure to Mtb, supporting an essential role for CD4+ T cells (Hirsch et al., J. Infect. Dis. 180:2069-2073, 1999). CD8+ T cells are also important for effective T cell immunity (see Lazarevic and Flynn, Am. J. Respir. Crit. Care Med. 166:1116-1121, 2002). In humans, Mtb-specific CD8+ T cells have been identified in Mtb-infected individuals and include CD8+ T cells that are both classically HLA-Ia restricted (see, for example, Lewinsohn et al., J. Immunol. 165:925-930, 2000) and nonclassically restricted by the HLA-Ib molecule HLA-E (Lewinsohn et al., J. Exp. Med. 187:1633-1640, 1998). However, there are no vaccines or therapeutic strategies that effectively induce an immune response, such as a CD8 response, to Mtb.
Diagnosis of tuberculosis is commonly achieved using a skin test, which involves intradermal exposure to tuberculin PPD (protein-purified derivative). Antigen-specific T cell responses result in measurable induration at the injection site by 48 to 72 hours after injection, which indicates exposure to Mycobacterial antigens. However, the sensitivity and specificity of this test are not ideal; individuals vaccinated with BCG cannot be distinguished from infected individuals. Furthermore, it is very difficult to diagnose TB in children because Mtb cannot be cultured from children in the majority of cases. In both children and adults, delays and missed diagnosis result in increased morbidity and mortality.